1. Field of the Invention
The present invention relates to a projection type image display apparatus such as a projector, and more particularly, to a projection type image display apparatus that allows autofocusing of a projection optical system.
2. Description of the Related Art
With regard to a projector, the center of a screen is generally not aligned with the optical axis of a projection lens considering that it is often the case that light is emitted from the projector on a desk onto the screen which is located a little higher than the desk. In this case, the position of the optical axis is shifted close to the bottom of the screen with respect to the center of the projection screen, which prevents trapezoidal distortion of the projected image and allows projection with an angle of elevation.
On the other hand, there is a projector which automatically performs focusing of a projection lens. In this case, a so-called passive AF (Auto Focus) using a two-image correlation or a so-called active AF, which emits infrared light onto a screen, receives the reflected light by a photoreception element and measures a distance from the photoreception output, is often used.
For example, in Japanese Patent No. 3,120,526, the passive AF is disclosed. The projector uses two optical systems aside from a projection lens which are separated in the length direction of a base line and photoreception sensors made up of photoelectric conversion elements for the respective optical systems. Then, a chart image is projected onto the center of the screen, the reflected light from the image pattern is received by a photoreception sensor to thereby obtain a parallax image signal and achieve focusing.
On the other hand, Japanese Patent No. 3,199,048 discloses an active AF. This projector projects infrared rays from an emission element onto the center of a screen and receives the reflected light by a photoreception sensor. Then, the focus lens is moved based on the photoreception output of the photoreception sensor to thereby achieve focusing.
However, since the focusing systems disclosed in the above described patents regard the center of the screen as the field of view of the sensor (detection range), in the case of a projection lens whose optical axis is shifted with respect to the center of the screen as described above, the orientation of the optical axis of the projection lens does not align with the orientation of the optical axis of the photoreception sensor. For this reason, the distance along the optical axis of the sensor is different from the distance along the optical axis of the projection lens, and as a result, it is not possible to achieve accurate focusing of the projection optical system.